You can have ten at a time. If one-third of the time is up and you decide that, for example, ten minutes of operation will last twenty seconds, then the next ten, then the next ten, you can have thirty in total.
If you focus only on the maximum, you will think that you can only have ten (which would be the case if you could not synchronize them and so could not exclude that they could all try to speak at the same time time. )
By calculating the average over the available time, you can achieve the same result without having to speculate on exactly how to plan it. During its uptime, each node transmits at an average of 3 + 1/3 Mbps. Divide 100 Mbps by this number gives 30.